Electronically erased D. E. Feldman Science 344, 1344 (2014); DOI: 10.1126/science.1255501
This copy is for your personal, non-commercial use only.
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of June 29, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/344/6190/1344.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/344/6190/1344.full.html#related This article cites 10 articles, 4 of which can be accessed free: http://www.sciencemag.org/content/344/6190/1344.full.html#ref-list-1 This article appears in the following subject collections: Physics http://www.sciencemag.org/cgi/collection/physics
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
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INSIGHTS | P E R S P E C T I V E S
spared. This preserved the essential TGF-β burst and the generation of an environment in vivo conducive to the development of autoantigen-specific Treg cells (in response to injection with autoantigen). This “reprogrammed” immune system could still respond to bacterial antigen, so overall immunity in the animal appears not to have been compromised. What is the true origin of the autoantigenspecific Treg cells that are responsible for the therapeutic effect? It is not known whether they derive from the distinct lineage of CD4 T cells from the thymus (FoxP3+Treg cells) (4–6, 11). Alternatively, they may derive from conventional peripheral CD4 T cells under the influence of TGF-β and the autoantigen (12). It is also not clear whether autoantigen-specific Treg cells can be sustained over time at sufficient amounts to ensure that the needed balance between “effectors” and “regulators” underlying immune tolerance is maintained. From a more practical point of view, therapeutic autoantigens for major autoimmune diseases (such as multiple sclerosis and autoimmune insulin–dependent diabetes) are available as therapeutic products and are safe (13). Also, monoclonal antibodies that recognize CD20 are likewise available and are safe (14, 15); CD8 monoclonal antibodies are still in development after many years. Aside from the scientific questions that must be addressed, there is the question of forging a strong academic-industry relationship if we want the strategy described by Kasagi et al. to become a clinical reality. It may be that the approach has implications far beyond autoimmune diseases, extending to the arenas of tolerance induction in transplantation, gene therapy, and regenerative medicine. ■ REFERENCES
1. R. H. Schwartz, Cold Spring Harb. Perspect. Biol. 4, a006908 (2012). 2. S. Kasagi et al., Sci. Transl. Med. 6, 241ra78 (2014). 3. J. F. Bach, N. Engl. J. Med. 347, 911 (2002). 4. J. D. Fontenot, M. A. Gavin, A. Y. Rudensky, Nat. Immunol. 4, 330 (2003). 5. J. D. Fontenot et al., Immunity 22, 329 (2005). 6. S. Sakaguchi et al., Immunol. Rev. 212, 8 (2006). 7. S. Sakaguchi, N. Sakaguchi, M. Asano, M. Itoh, M. Toda, J. Immunol. 155, 1151 (1995). 8. N. Marek-Trzonkowska, M. Myśliwec, J. Siebert, P. Trzonkowski, Pediatr. Diabetes 14, 322 (2013). 9. Q. Tang, J. A. Bluestone, Cold Spring Harb. Perspect. Med.3, a015552 (2013). 10. S. Perruche et al., Nat. Med. 14, 528 (2008). 11. J. A. Bluestone, A. K. Abbas, Nat. Rev. Immunol. 3, 253 (2003). 12. W. Chen et al., J. Exp. Med. 198, 1875 (2003). 13. J. Ludvigsson et al., N. Engl. J. Med. 359, 1909 (2008). 14. S. L. Hauser et al., N. Engl. J. Med. 358, 676 (2008). 15. M. D. Pescovitz et al., N. Engl. J. Med. 361, 2143 (2009).
10.1126/science.1256864
1344
PHYSICS
Electronically erased Erasing knowledge of a quantum system changes its state By D. E. Feldman
T
he fate of Schrödinger’s cat depends on the particular path of a single electron. If the electron hits the trigger, which opens a bottle of poisonous gas, then the cat dies. If the path misses the trigger, the cat lives. According to quantum mechanics, electrons do not normally follow a definite trajectory. Instead, an electron can trace both paths at the same time. As a result, Schrödinger’s cat is both dead and alive. Quantum mechanics also teaches us that a “which-path” detector—that is, any measurement device that can show where the electron travels—forces the electron to choose just one path, thus sealing the cat’s destiny. But what will happen after the information collected by the
A
Arm 1 D1 System D2
S1
when confined to two dimensions in a very pure semiconductor in a strong magnetic field. This is known as the quantum Hall effect. In particular, transport is one-dimensional, with electrons following just a small set of paths determined by metallic gates on top of the sample. This property was used by Weisz et al. to define the allowed trajectories for electrons in the experiment (see the figure, panel A). The experimental setup consists of two identical devices, known as electronic MachZehnder interferometers (3). One of them plays the role of a quantum system under investigation; the other is a detector (see the figure, panel A). To understand the experiment, we first need to address how a single interferometer works. Electrons depart from terminal S1 along the upper and lower arms
B Eraser on
Arm 2 +
S3
Arm 3 Detector
D3
Eraser of
D4 Arm 4 Electronic quantum eraser. (A) A schematic of the device. Electrons follow arms 1 to 4 in the directions of the arrows. Electrons in arm 2 repel electrons in arm 3. (B) The eraser effect on the quantum interference of matter waves.
detector has been erased? The results of a semiconductor version of such an experiment, reported by Weisz et al. on page 1363 of this issue (1), suggest that the cat will come back to the middle ground between life and death. In contrast to classical physics, any measurement on a quantum system affects its behavior. Hence, the properties of the system depend on what questions one asks. For example, according to the uncertainty principle, it is meaningful to ask about a position or velocity of a quantum particle, but it cannot have a definite position and velocity at the same time. Weisz et al. add another twist: What happens if a question is asked but the answer (that is, the measurement result) is erased? To address this problem, Weisz et al. have realized an electronic quantum eraser in a semiconductor nanostructure as proposed by Kang (2). Electrons exhibit rich behavior
1 and 2 and are detected in terminals D1 and D2. If electrons were classical particles, the number of arrivals to D2 would simply be determined by the incoming current at S1. The wave-particle duality of quantum mechanics implies more complexity: Electrons propagate along paths 1 and 2 in the form of waves. The waves interfere where the two arms meet (essentially, the oscillations in the two waves add up). Hence, the output signal at D2 depends on the phase difference between the waves. The latter is controlled by the area between the device arms. Thus, the number of electron arrivals to D2 oscillates as a function of the area. In the two-interferometer system, electric forces result in repulsion between the electrons in the lower arm of the quantum system and those in the upper arm of the deDepartment of Physics, Brown University, Providence, RI 02912, USA. E-mail: [email protected]
sciencemag.org SCIENCE
20 JUNE 2014 • VOL 344 ISSUE 6190
Published by AAAS
tector. Also, the upper arm of the quantum system is too far to have much effect on the lower interferometer. Thus, the electric current in the detector depends on the paths of the electrons in the upper interferometer. In other words, the detector reads the which-path information. Such a measurement disturbs the quantum system and suppresses interference between arms 1 and 2. As a result, the current oscillations in D2 are reduced. The experimentally measured correlation between the currents in D2 and D4 also shows reduced oscillations (see the figure, panel B). The last important piece of the physics is interference between arms 3 and 4 in the detector. By tuning the magnetic field, Weisz et al. reach the regime where the interference effect makes the electric current in D4 almost independent of the which-path information about the upper interferometer. Thus, they erase the data, detected by arm 3, before it can be read out. What is the effect on the quantum system? The oscillations in the current correlation between D2 and D4 return in full force: Schrödinger’s cat is brought back to the middle ground between life and death. Quantum eraser experiments were previously performed in an optical setting with photons (4–6). The electronic setting of Weisz et al. brings in an additional advantage due to Coulomb interaction. In contrast to photons, electrons strongly interact, and this opens ways to manipulate and entangle them. In a striking difference from optical experiments, the quantum system and the detector used by Weisz et al. are identical. Essentially, two Schrödinger’s cats are staring at each other. A possible extension of the experiment would be a delayed-choice experiment (7–10). Weisz et al. erase the which-path information immediately after it is obtained. But what if the information is stored for some time before being deleted? Would this revive a cat that has long been dead? ■ REFERENCES AND NOTES
1. 2. 3. 4. 5. 6. 7.
PHOTO: PETER DE KNIJFF
8. 9. 10.
E. Weisz et al., Science 344, 1363 (2014). K. Kang, Phys. Rev. B 75, 125326 (2007). Y. Ji et al., Nature 422, 415 (2003). M. O. Scully, K. Drühl, Phys. Rev. A 25, 2208 (1982). P. G. Kwiat, A. M. Steinberg, R. Y. Chiao, Phys. Rev. A 45, 7729 (1992). T. J. Herzog, P. G. Kwiat, H. Weinfurter, A. Zeilinger, Phys. Rev. Lett. 75, 3034 (1995). Y.-H. Kim, R. Yu, S. P. Kulik, Y. Shih, M. O. Scully, Phys. Rev. Lett. 84, 1 (2000). X. S. Ma et al., Proc. Natl. Acad. Sci. U.S.A. 110, 1221 (2013). A. Peruzzo, P. Shadbolt, N. Brunner, S. Popescu, J. L. O’Brien, Science 338, 634 (2012). F. Kaiser, T. Coudreau, P. Milman, D. B. Ostrowsky, S. Tanzilli, Science 338, 637 (2012).
ACKNOWLEDGMENTS
Supported by NSF grant DMR-1205715. 10.1126/science.1255501
GENETICS
How carrion and hooded crows defeat Linnaeus’s curse How two crow species maintain their identity raises questions about species concepts By Peter de Knijff
E
ver since the carrion crow (Corvus corone) and the hooded crow (Corvus cornix) were described by Linnaeus as two species in 1758 (1), their taxonomic status has been debated. These two phenotypically distinct crow taxa have a Palearctic breeding distribution with two stable zones of hybridization, one of which runs roughly north to south through central Europe (2) (see the figure). Primarily based on lack of complete reproductive isolation and because of genomewide genetic homogeneity, they are often considered to represent two subspecies of the carrion crow (3, 4). Some researchers, however, elevated these two taxa to full species in 2003 (5), a proposal supported by apparent nonrandom mating and reduced hybrid fitness. Earlier work (4) suggested that differences in gene expression, despite the lack of genomic nucleotide divergence, could serve as a sensitive Carrion crow indicator of speciation, although the exact mechanisms driving this process remained unknown. On page 1410 of this issue, Poelstra et al. (6) show, using a speciation genomics approach (7), how differential gene expression of only a small proportion (
This copy is for your personal, non-commercial use only.
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of June 29, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/344/6190/1344.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/344/6190/1344.full.html#related This article cites 10 articles, 4 of which can be accessed free: http://www.sciencemag.org/content/344/6190/1344.full.html#ref-list-1 This article appears in the following subject collections: Physics http://www.sciencemag.org/cgi/collection/physics
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on June 29, 2014
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here.
INSIGHTS | P E R S P E C T I V E S
spared. This preserved the essential TGF-β burst and the generation of an environment in vivo conducive to the development of autoantigen-specific Treg cells (in response to injection with autoantigen). This “reprogrammed” immune system could still respond to bacterial antigen, so overall immunity in the animal appears not to have been compromised. What is the true origin of the autoantigenspecific Treg cells that are responsible for the therapeutic effect? It is not known whether they derive from the distinct lineage of CD4 T cells from the thymus (FoxP3+Treg cells) (4–6, 11). Alternatively, they may derive from conventional peripheral CD4 T cells under the influence of TGF-β and the autoantigen (12). It is also not clear whether autoantigen-specific Treg cells can be sustained over time at sufficient amounts to ensure that the needed balance between “effectors” and “regulators” underlying immune tolerance is maintained. From a more practical point of view, therapeutic autoantigens for major autoimmune diseases (such as multiple sclerosis and autoimmune insulin–dependent diabetes) are available as therapeutic products and are safe (13). Also, monoclonal antibodies that recognize CD20 are likewise available and are safe (14, 15); CD8 monoclonal antibodies are still in development after many years. Aside from the scientific questions that must be addressed, there is the question of forging a strong academic-industry relationship if we want the strategy described by Kasagi et al. to become a clinical reality. It may be that the approach has implications far beyond autoimmune diseases, extending to the arenas of tolerance induction in transplantation, gene therapy, and regenerative medicine. ■ REFERENCES
1. R. H. Schwartz, Cold Spring Harb. Perspect. Biol. 4, a006908 (2012). 2. S. Kasagi et al., Sci. Transl. Med. 6, 241ra78 (2014). 3. J. F. Bach, N. Engl. J. Med. 347, 911 (2002). 4. J. D. Fontenot, M. A. Gavin, A. Y. Rudensky, Nat. Immunol. 4, 330 (2003). 5. J. D. Fontenot et al., Immunity 22, 329 (2005). 6. S. Sakaguchi et al., Immunol. Rev. 212, 8 (2006). 7. S. Sakaguchi, N. Sakaguchi, M. Asano, M. Itoh, M. Toda, J. Immunol. 155, 1151 (1995). 8. N. Marek-Trzonkowska, M. Myśliwec, J. Siebert, P. Trzonkowski, Pediatr. Diabetes 14, 322 (2013). 9. Q. Tang, J. A. Bluestone, Cold Spring Harb. Perspect. Med.3, a015552 (2013). 10. S. Perruche et al., Nat. Med. 14, 528 (2008). 11. J. A. Bluestone, A. K. Abbas, Nat. Rev. Immunol. 3, 253 (2003). 12. W. Chen et al., J. Exp. Med. 198, 1875 (2003). 13. J. Ludvigsson et al., N. Engl. J. Med. 359, 1909 (2008). 14. S. L. Hauser et al., N. Engl. J. Med. 358, 676 (2008). 15. M. D. Pescovitz et al., N. Engl. J. Med. 361, 2143 (2009).
10.1126/science.1256864
1344
PHYSICS
Electronically erased Erasing knowledge of a quantum system changes its state By D. E. Feldman
T
he fate of Schrödinger’s cat depends on the particular path of a single electron. If the electron hits the trigger, which opens a bottle of poisonous gas, then the cat dies. If the path misses the trigger, the cat lives. According to quantum mechanics, electrons do not normally follow a definite trajectory. Instead, an electron can trace both paths at the same time. As a result, Schrödinger’s cat is both dead and alive. Quantum mechanics also teaches us that a “which-path” detector—that is, any measurement device that can show where the electron travels—forces the electron to choose just one path, thus sealing the cat’s destiny. But what will happen after the information collected by the
A
Arm 1 D1 System D2
S1
when confined to two dimensions in a very pure semiconductor in a strong magnetic field. This is known as the quantum Hall effect. In particular, transport is one-dimensional, with electrons following just a small set of paths determined by metallic gates on top of the sample. This property was used by Weisz et al. to define the allowed trajectories for electrons in the experiment (see the figure, panel A). The experimental setup consists of two identical devices, known as electronic MachZehnder interferometers (3). One of them plays the role of a quantum system under investigation; the other is a detector (see the figure, panel A). To understand the experiment, we first need to address how a single interferometer works. Electrons depart from terminal S1 along the upper and lower arms
B Eraser on
Arm 2 +
S3
Arm 3 Detector
D3
Eraser of
D4 Arm 4 Electronic quantum eraser. (A) A schematic of the device. Electrons follow arms 1 to 4 in the directions of the arrows. Electrons in arm 2 repel electrons in arm 3. (B) The eraser effect on the quantum interference of matter waves.
detector has been erased? The results of a semiconductor version of such an experiment, reported by Weisz et al. on page 1363 of this issue (1), suggest that the cat will come back to the middle ground between life and death. In contrast to classical physics, any measurement on a quantum system affects its behavior. Hence, the properties of the system depend on what questions one asks. For example, according to the uncertainty principle, it is meaningful to ask about a position or velocity of a quantum particle, but it cannot have a definite position and velocity at the same time. Weisz et al. add another twist: What happens if a question is asked but the answer (that is, the measurement result) is erased? To address this problem, Weisz et al. have realized an electronic quantum eraser in a semiconductor nanostructure as proposed by Kang (2). Electrons exhibit rich behavior
1 and 2 and are detected in terminals D1 and D2. If electrons were classical particles, the number of arrivals to D2 would simply be determined by the incoming current at S1. The wave-particle duality of quantum mechanics implies more complexity: Electrons propagate along paths 1 and 2 in the form of waves. The waves interfere where the two arms meet (essentially, the oscillations in the two waves add up). Hence, the output signal at D2 depends on the phase difference between the waves. The latter is controlled by the area between the device arms. Thus, the number of electron arrivals to D2 oscillates as a function of the area. In the two-interferometer system, electric forces result in repulsion between the electrons in the lower arm of the quantum system and those in the upper arm of the deDepartment of Physics, Brown University, Providence, RI 02912, USA. E-mail: [email protected]
sciencemag.org SCIENCE
20 JUNE 2014 • VOL 344 ISSUE 6190
Published by AAAS
tector. Also, the upper arm of the quantum system is too far to have much effect on the lower interferometer. Thus, the electric current in the detector depends on the paths of the electrons in the upper interferometer. In other words, the detector reads the which-path information. Such a measurement disturbs the quantum system and suppresses interference between arms 1 and 2. As a result, the current oscillations in D2 are reduced. The experimentally measured correlation between the currents in D2 and D4 also shows reduced oscillations (see the figure, panel B). The last important piece of the physics is interference between arms 3 and 4 in the detector. By tuning the magnetic field, Weisz et al. reach the regime where the interference effect makes the electric current in D4 almost independent of the which-path information about the upper interferometer. Thus, they erase the data, detected by arm 3, before it can be read out. What is the effect on the quantum system? The oscillations in the current correlation between D2 and D4 return in full force: Schrödinger’s cat is brought back to the middle ground between life and death. Quantum eraser experiments were previously performed in an optical setting with photons (4–6). The electronic setting of Weisz et al. brings in an additional advantage due to Coulomb interaction. In contrast to photons, electrons strongly interact, and this opens ways to manipulate and entangle them. In a striking difference from optical experiments, the quantum system and the detector used by Weisz et al. are identical. Essentially, two Schrödinger’s cats are staring at each other. A possible extension of the experiment would be a delayed-choice experiment (7–10). Weisz et al. erase the which-path information immediately after it is obtained. But what if the information is stored for some time before being deleted? Would this revive a cat that has long been dead? ■ REFERENCES AND NOTES
1. 2. 3. 4. 5. 6. 7.
PHOTO: PETER DE KNIJFF
8. 9. 10.
E. Weisz et al., Science 344, 1363 (2014). K. Kang, Phys. Rev. B 75, 125326 (2007). Y. Ji et al., Nature 422, 415 (2003). M. O. Scully, K. Drühl, Phys. Rev. A 25, 2208 (1982). P. G. Kwiat, A. M. Steinberg, R. Y. Chiao, Phys. Rev. A 45, 7729 (1992). T. J. Herzog, P. G. Kwiat, H. Weinfurter, A. Zeilinger, Phys. Rev. Lett. 75, 3034 (1995). Y.-H. Kim, R. Yu, S. P. Kulik, Y. Shih, M. O. Scully, Phys. Rev. Lett. 84, 1 (2000). X. S. Ma et al., Proc. Natl. Acad. Sci. U.S.A. 110, 1221 (2013). A. Peruzzo, P. Shadbolt, N. Brunner, S. Popescu, J. L. O’Brien, Science 338, 634 (2012). F. Kaiser, T. Coudreau, P. Milman, D. B. Ostrowsky, S. Tanzilli, Science 338, 637 (2012).
ACKNOWLEDGMENTS
Supported by NSF grant DMR-1205715. 10.1126/science.1255501
GENETICS
How carrion and hooded crows defeat Linnaeus’s curse How two crow species maintain their identity raises questions about species concepts By Peter de Knijff
E
ver since the carrion crow (Corvus corone) and the hooded crow (Corvus cornix) were described by Linnaeus as two species in 1758 (1), their taxonomic status has been debated. These two phenotypically distinct crow taxa have a Palearctic breeding distribution with two stable zones of hybridization, one of which runs roughly north to south through central Europe (2) (see the figure). Primarily based on lack of complete reproductive isolation and because of genomewide genetic homogeneity, they are often considered to represent two subspecies of the carrion crow (3, 4). Some researchers, however, elevated these two taxa to full species in 2003 (5), a proposal supported by apparent nonrandom mating and reduced hybrid fitness. Earlier work (4) suggested that differences in gene expression, despite the lack of genomic nucleotide divergence, could serve as a sensitive Carrion crow indicator of speciation, although the exact mechanisms driving this process remained unknown. On page 1410 of this issue, Poelstra et al. (6) show, using a speciation genomics approach (7), how differential gene expression of only a small proportion (
